Hydrophilic poly(triphenylamines) with phosphonate groups on the side chains: synthesis and photovoltaic applications

Abstract

Two triphenylamine-based homopolymers PTPA-EP and PTPA-PO₃Na₂, comprising diethyl phosphonate and sodium phosphonate end groups on side chains, respectively, were synthesized. The UV-vis absorption and photoluminescence (PL) properties of the PTPA-EP and PTPA-PO₃Na₂ are mainly determined by the conjugated poly(triphenylamine) main chain. The PTPA-EP and PTPA-PO₃Na₂ possess comparable HOMO levels of around −5.03 eV. The PTPA-EP, with better solubility than PTPA-PO₃Na₂ in hydrophilic solvents, was utilized as cathode interlayer to construct efficient bulk-heterojunction photovoltaic cells with a low bandgap poly(2,7-carbazole) (PCDTBT) as the polymer donor and [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) as the acceptor. The work function of ITO was shifted to −4.3 eV by PTPA-EP, which matches well with the LUMO level of PC₇₁BM for good electron extraction. Inverted solar cells with a device configuration of ITO/PTPA-EP/active layer/MoO₃/Al exhibited a power conversion efficiency (PCE) of 4.59%, which is a good efficiency among inverted solar cells with an organic interlayer on an ITO cathode. The PCE shows a 79% increase in comparison to that of a bare ITO cathode, though the efficiency is lower than 5.13% for an inverted solar cell with an inorganic ZnO interlayer on ITO. Moreover, a conventional solar cell with a device configuration of ITO/PEDOT:PSS/active layer/PTPA-EP/Al could show a better PCE of 5.27%. The results indicate that PTPA-EP is a promising new cathode interlayer for high efficiency inverted and conventional solar cells.